Disc drive storage capacities are continuing to increase dramatically, resulting in part from rapid advances in on-disc coding schemes and magnetic sensitivities of read/write head components. Data (including servo data) is magnetically stored on concentric tracks patterned on the disc. The increase in track density on the disc relates to a corresponding decrease in the width of the read and write heads (e.g., transducer heads). As areal densities increase, previously negligible position error effects become more prominent. Therefore, positioning a transducer head precisely within a track becomes increasingly important and difficult. Accordingly, improved precision is required in the testing of data storage disc drive components to verify acceptable operation at these higher densities.
To adequately test the components of a data storage disc drive (e.g., read heads, head gimbal assemblies (HGAs), servo control schemes), therefore, test equipment must be improved to provide the precision needed to test these improved data storage disc drives. In fact, it is axiomatic that the precision of test equipment must exceed that of the improved data storage disc drives that it tests. The electrical characteristics of a read head, for example, are often evaluated on a high precision electrical tester called a “spin-stand”. A spin-stand is designed for enhanced stability and typically includes a spindle on which a data storage disc is rotated at high speeds (e.g., 3,600 to 15,000 RPM, although spin-stands are known to achieve higher revolution speeds, such as 30,000 RPM in some current models) and a motion platform that positions a read/write head relative to the data storage disc rotating on the spindle. The motion platform usually includes a coarse positioning stage and a micropositioning stage. Both the spindle motor and the motion platform are securely mounted on a support base or stable surface. Coarse positioning is commonly implemented using X and Y oriented linear motors mounted to a stable surface. The micropositioning stage commonly includes a piezoelectric actuator and moves along a single axis under control of a higher precision linear motor. An incremental encoder or capacitive sensor responds to the linear position of the micropositioning stage to indicate the position of a transducer head mounted on the micropositioning stage.
However, even the existing two stage motion platforms cannot provide the needed precision and movement for higher density disc drives. First, it is desirable that the micropositioning stage also has a high servo bandwidth to better reject external disturbances (i.e., background vibrations, spindle excitation, and windage) and to hold the head on a track having high frequency irregularities. For example, to maintain proper track following of a higher density data storage disc, a micropositioning stage may have to change the radial position of the head multiple times during a single disc rotation. A positioning staging having this capability is referred to herein as having “high bandwidth.” However, piezoelectric actuators used in existing spin stands cannot compensate for such high frequency variations observed in higher density data storage disc drives.
Second, it is also desirable for the read head to follow prewritten servo tracks recorded on the storage disc. Such operation is referred to as “track following.” Existing motion platforms may be adequate for following ideally (or almost ideally) circular recorded tracks, but prewritten tracks on higher density data storage discs will typically be too eccentric with respect to the spindle of the spin-stand (e.g., due to tolerances in the spindle clamp). That is, the path that the head travels as it follows a track is not likely to be precisely circular. The repeatable runout resulting from this eccentricity can be as high as 60–90 tracks on such data storage discs. Therefore, the micropositioning stage must have a large enough stroke to follow a track having a high amplitude variation from the circular ideal. As the head follows a track around a disc rotation, the micropositioning stage must be able to vary the radial position of the head substantially to accommodate the magnitude of circular irregularities in the track. A positioning stage having this capability is referred to herein as having a “large stroke.” However, piezoelectric actuators presently used in existing spin stands cannot support such large amplitude irregularities observed in higher density data storage disc drives.
Accordingly there is a need for a high bandwidth, large stroke spin-stand for testing components of data storage disc drives.